This invention relates to the sampling of waste gas (also known as "exhaust gas", or "off-gas") from an industrial furnace, particularly but not exclusively that from an electric arc furnace (EAF). More specifically the invention concerns a probe for carrying out such sampling, and methods relating thereto.
In many industrial furnace operations, and in particular in EAF steelmaking, large volumes of carbon monoxide and/or other combustible gases are evolved during certain parts of the process cycle. These combustible gases form a significant proportion of the waste gas leaving the furnace, along with a normally large amount of particulate dust and liquid or semi-liquid slag droplets.
It is usual to try and burn the combustible gases in the furnace, since this recovers part of the heat content of the gases, and thus saves energy. It also improves the nature of the gases eventually discharged to the environment by destroying toxic gases, and it helps protect downstream waste gas treatment equipment by combusting any explosive gases. To burn the combustible gases, oxygen is lanced into the furnace, and in order to determine the initiation, duration and rate of such oxygen supply it is necessary continuously to analyze the waste gas (by measuring its oxygen or carbon monoxide content, for example) and to use the resulting measurement to open or modulate oxygen admission valves. Because these systems operate on a feedback principle, they must be quick-reacting to any change in the waste gas composition. This means that analysis of the waste gas is carried out on a sample of waste gas drawn from a position as close to the furnace as is practicable, usually at the point where the fixed ducting for drawing the waste gas from the furnace abuts the movable upper section of the furnace. In a typical EAF at this position, the waste gas temperature can reach about 1600.degree. C.
In order to obtain a representative sample of waste gas, the sample is usually drawn from the waste gas flow at a position towards the fastest-flowing part of the waste gas, where the boundary layer effects of the duct walls on the waste gas flow, and its composition, are negligible. This position is generally in the unburned part of the flame caused by the waste gas combustion as it enters the duct from the furnace. The sample is generally obtained by means of a longitudinal probe which is normally inserted vertically downwardly so that its inlet end is located in the above-described position.
The waste gas at the position from which samples are normally taken entrains a high concentration of solid, liquid and semi-liquid contaminants (hereinafter "particulate matter"--slag and the like) which must be separated from the sample of waste gas drawn off before its composition can be analyzed. The particulate matter also impacts on the probe and tends to agglomerate thereon, in particular adjacent the inlet end of the probe, often to the extent that the inlet to the probe becomes blocked, necessitating replacement and/or cleaning of the probe. Particulate matter entrained in the waste gas sample drawn into the probe can also agglomerate inside the probe (at cold spots, or where there is stagnant flow, for example) which again can block the probe. Blockage of the probe, both internal and external, can be exacerbated by the common practice of cooling the probe; it is commonly thought essential to cool the probe, usually with a water cooling system, so as to ameliorate the adverse effects of the high ambient temperature (in the waste gas) on the probe, but cooling the probe can also increase the tendency of molten droplets impacting on the probe to solidify and agglomerate.
The provision of waste gas sampling means which are both capable of delivering a representative sample of waste gas, uncontaminated by entrained particulate matter, for analysis and also not susceptible to blockage by the agglomeration of liquid, semi-liquid and/or solid particulate matter entrained in the waste gas flow is a long-standing problem. It is a particular problem in applications (such as in some EAFs) where there is a large amount of particulate matter entrained in the waste gas flow. Various designs of water cooled probes have been proposed, but none have satisfactorily managed to provide a clean sample of waste gas whilst resisting becoming blocked over an extended period--in most cases, there has been a tendency for blockages to arise, and/or the waste gas analysis to become inaccurate, after only a few days' usage.